This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The low-temperature group at LASSP is currently actively studying impurity-helium solids. Impurity-helium (Im-He) solids are porous solids created by injecting a mixed beam of helium gas and an impurity gas or impurity gases into superfluid 4He [1]. The impurities or clusters of impurities from the beam are then surrounded by a thin layer of solid helium. Van der Waals forces bind these components into a tenuous, aerogel-like structure with nanoscale pores. So far, CW ESR has been the main tool used to study Im-He solids. Pulsed ESR methods promise to provide additional details of the structure of these fragile objects. For example, for nitrogen atoms, ESEEM from nearby nitrogen nuclei can be detected to report on distances and the number of surrounding nuclei. The same holds true for H and D atoms. The LASSP group undertook the construction of a pulse ESR spectrometer with help from ACERT. ACERT has provided the necessary level of assistance including consultation on spectrometer design, providing some critical components, such as a TWT amplifier, and helping with the construction of the microwave bridge. The microwave pulse bridge construction was completed. The bridge was tested and found to be ready for integration with the rest of the spectrometer.